Conventional blowers used for a continuous positive airway pressure therapy are shown in FIGS. 4 and 5 as examples. In each of the blowers, a scroll case 53 is constituted by a first part 51 and a second part 52, and a motor M and a rotatable impeller 55 are provided in the case 53.
A bearing housing 56, which is formed into a cylindrical shape, is attached to the first part 51. A motor circuit board 57 and a stator 58 are attached to the bearing housing 56. A rolling bearing, e.g., a ball bearing 59 shown in FIG. 4, or a slide bearing, e.g., an oil retaining bearing 60 shown in FIG. 5, which receives a radial load (a load in a radial direction of a rotor shaft 61) and a thrust load (a load in an axial direction of the rotor shaft 61), is attached in the bearing housing 56 so as to rotatably hold the rotor shaft 61. The impeller 55 is fixed to the rotor shaft 61 by, for example, molding, adhesive bonding or press fitting. A rotor 62 is constituted by a cup-shaped rotor yoke and a ring magnet (not shown), which is fixed on an inner circumferential face of the rotor yoke and which faces pole teeth of a stator.
The impeller 55 is provided on the second part 52 side. An inlet 52a, from which a fluid (air) is sucked, is formed at a center of the second part 52. The impeller 55 is attached to the rotor shaft 61, and blades 55a are faced toward the inlet 52a. A compression chamber 63 is formed around the impeller 55. An outlet (not shown), which is communicated with the compression chamber 63, is extended from the second part 52 in a tangential direction.
By stating the motor M, the impeller 55 is rotated together with the rotor 62 and the fluid is sucked from the inlet 52a. The sucked fluid is introduced radially outward, by the blades 55a, and compressed in the compression chamber 63. Then, the compressed fluid is discharged from the outlet. The above described blower is used as, for example, a turbine of a breathing assistance unit (see Japanese Patent No. 4159992).
In the above describe blower, the fluid is sucked from the inlet 52a formed at a center of the impeller 55, so that negative pressure is produced in the vicinity of the inlet 52a. On the other hand, positive pressure is produced in the vicinity of the outlet and on the rear sides of the impeller 55. Therefore, a lifting force F, which lifts the impeller 44 from the motor M side toward the blade 55a side, is applied to the impeller 55. The lifting force F acting on the impeller 55 is increased by increasing the fluid pressure produced by the impeller 55, and the force F sometimes exceeds an attractive force generated between the rotor magnet and the pole teeth of the stator.
Thus, interference between the impeller 55 and the second part 52 must be prevented. In case of using the ball bearing shown in FIG. 4 as the rolling bearing, the rotor shaft 61 is supported in the thrust direction, so the lifting force F acting on the impeller 55 causes no problems.
However, the rolling bearing, e.g., ball bearing, capable of receiving the loads in both of the radial direction and the thrust direction is expensive.
Further, in case of using the slide bearing shown in FIG. 5, e.g., oil retaining bearing, the slide bearing can receive the load in the radial direction, but cannot receive the load in the thrust direction. Thus, as shown in FIG. 5, the rotor shaft 61 is retained by retaining means, e.g., a snap ring 64.
However, in case that the rotor is rotated at a high speed or the lifting force F is too great, the retaining means will be abraded in a short time.
In case that a movement of the impeller 55 in the axial direction is limited by a thrust receiving member which prevents the interference between the impeller 55 and the second part 52, the thrust receiving member can be provided on the motor M side. But, the inlet 52a is formed in the impeller 55 side, so the thrust receiving member cannot be provided on the impeller 55 side.